I'm currently looking into an cheap InfiniBand solution and it seems like used 20 Gigabit/s host cards and switches are rather cheap to get but I'm getting really confused when it comes to cables as the prices are differing a lot and their tech description differs as well (if they even have any, many that I found just have a part number and the length).

I couldn't find any ressources that explain the diffrences (if there are any). I found cables that are supposed to connect to SAS/S-ATA which sounded very weird to me and it seems that there is some sort of quality difference.

So, what do I need to look for if I want a cable that would connect a host card and switch that support 20Gbit/s (both card and switch that I looked into a little closer have a "4x DDR" spec but I'm not exactly sure what that tells me, DDR as in DDR memory?).

Physical Interconnection

InfiniBand uses copper CX4 cable for SDR and DDR rates — also commonly used to connect SAS (Serial Attached SCSI) HBAs to external (SAS) disk arrays. With SAS, this is known as an SFF-8470 connector, and is referred to as an "InfiniBand-style" Connector. The latest connectors used with QDR and FDR are QSFP (Quad SFP) and can be copper or fiber, depending on the length required.